In my situation, I was using CVS for a friends manuscript. Seeing that she is new to GNU/Linux in general, and I had already convinced her to use Vim and LaTeX for writing out her story, I felt that CVS would have been much more easier for her to learn.
For her, she could examine her changes easily within KDE's built-in Cervisia option.
The commands are simpler using CVS. For example under CVS, to checkout a copy of your work, you would issue;

cvs -d ~/cvsroot checkout project-files

Under subversion it is not difficult, but just longer.

svn checkout file:///home/locustfurnace/subversion/trunk project-files

Another reason was that there are more available front-ends (including Win32) for accessing CVS than Subversion. Plus, there are some filters for LaTeX for handling CVS comments.

If I recall correctly, I was not particularly a fan of the revision numbering scheme. I think subversion incremented revisions on a whole numbering system, such as 1, 2, 3, 4 while I had the ability in CVS to use 1.1.1.2, 1.1.1.3, 1.1.1.4. If necessary.

It's usually the little things which become a deciding factor for myself in what way I plan to go when picking software. The same reasons why I decided to chose LaTeX over DOCBOOK or XML when writing out a novel.
As stated earlier, if your controlling your own project, and starting from scratch. I think Subversion may have more options for you. But if your collaborating on a project, then you should factor in how those others users will be accessing the repository, and what tools they are familiar with.